1. Field of the Invention
This invention relates to a technique of automatically compensating for waveform distortion of a televison signal, and more particularly to a system for automatically compensating for waveform distortion of a television signal occurring in the television signal transmission path including the internal circuits of a television receiver.
2. Description of the Prior Art
Typical waveform distortion of a television signal includes overshoot, smearing, ringing and ghost appearing at rising and falling portions of the waveform, and such waveform distortion is undesirable in that the quality of reproduced pictures is extremely deteriorated when the waveform distortion is excessive. The sources of such undesirable waveform distortion include multiplex propagation of the waves due to reflection by buildings and mountains, impedance mismatching between the receiving antenna and the feeder (or cable), impedance mismatching between the feeder (or calbe) and the television receiver, and deviation of the frequency characteristic of the filter in the intermediate frequency amplifier circuit or the like in the television receiver from the most suitable operating condition.
Generally, the waveform distortion due to these sources is not non-linear distortion but so-called linear distortion caused merely by the deviation of the frequency characteristic of the filter from the desired one. Therefore, the waveform distortion of the kind above described can be compensated as a matter of principle by inserting in the television signal transmission path a filter having a frequency characteristic which is exactly opposite to the frequency characteristic giving rise to the undesirable waveform distortion.
A transversal filter can be used for compensation for such waveform distortion and is now practically employed as a waveform equalizer. Such a waveform equalizer suitable for the equalizing purpose for a video signal can be composed by a delay circuit having a plurality of taps each giving a delay time of a .tau. seconds, a plurality of adjusting circuits capable of adjusting the polarity and amplitude of the outputs of the individual taps independently of each other, and an adder circuit for adding the outputs of these adjusting circuits.
The principle of waveform equalization is based on the theory that a given transmission characteristic 1/8(.omega.) can be expanded to a polynominal given by ##EQU1## Further, form another point of view or when considered in regard to the time axis, a waveform H(t) can be obtained by integrating an echo of a given input waveform X(t) as follows: ##EQU2## In the two equations above described, Ck is a coefficient used for adjusting the polarity and amplitude of the outputs of the individual taps of the delay circuit by the adjusting circuits, and caleed a tap coefficient in this specification. These tap coefficients Ck are suitably adjusted by transmitting a test waveform such as a sine squared pulse waveform which facilitates distortion detection and manually controlling the individual adjusting circuits while observing the output waveform of the equalizer so that the overall distortion of the transmitted waveform can be reduced to a minimum. Such manner of adjustment is easily feasible in industrial television equipments such as those installed in broadcasting stations. However, it is almost impossible for the users of home television receivers to make such adjustment.
A method for automatically attaining such waveform equalization has been proposed hitherto. This proposed method comprises transmitting and receiving a predetermined test waveform prior to the transmission of a desired signal and detecting distortion of the test waveform for automatically equalizing the waveform. Such a method is described, for example, in a paper of R. W. Lucky and H. R. Rudin entitled "An Automatic Equalizer for General-Purpose Communication Channels" B.S.T.J., Vol. 46, November, 1967, pp. 2179-2208. However, the disclosed automatic waveform equalization is only applicable to a communication system in which a transmitting station and a receiving station have a specific predetermined relationship therebetween, due to the fact that there is the necessity for transmission and reception of a special test waveform. Another applicable case is such that the waveform of a transmitted signal is always constant like the waveform used in a digital signal communication system. Therefore, the disclosed automatic waveform equalization is not applicable to a system such as a television broadcasting system in which the waveform of the transmitted signal varies incessantly and the signal is received by many receivers of various kinds.